[lworld] RFR: 8371604: [lworld] SIGSEGV in InlineKlass::is_payload_marked_as_null

[Paul Hübner]

Hi all,

This patch fixes several `int` overflows relating to flat array element loads/stores. 
I've included some gtests that address a subset of these changes.  We should test flat arrays larger than 16GB in size, which is not something we can feasibly do in our tier1 jtreg tests. I've filed [JDK-8372261](https://bugs.openjdk.org/browse/JDK-8372261) as a follow-up RFE to investigate this.

Testing: tiers 1-4, [`LocalityCheck.java`](https://bugs.openjdk.org/secure/attachment/117136/LocalityCheck.java) run locally 100 times.

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Commit messages:
 - Fix up tests.
 - Include test case.
 - Fix another overflow.
 - Use ptrdiff_t in value_at_addr.

Changes: https://git.openjdk.org/valhalla/pull/1752/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1752&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371604
  Stats: 74 lines in 4 files changed: 69 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/valhalla/pull/1752.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1752/head:pull/1752

PR: https://git.openjdk.org/valhalla/pull/1752